An inherent limitation of conventional printed circuit boards is the tendency of microstrip transmission lines to allow escape of electromagnetic radiation into the environment. This is very undesirable and can cause interference with radio and television operations, for example. Also, when microstrip transmission lines are located close to each other on a printed circuit board, electromagnetic fields in adjacent microstrips can become coupled, i.e, there is crosstalk between adjacent microstrips. This is also very undesirable.
Basically, any electromagnetic interference with the signal in a microstrip transmission line is undesirable and can amount to more than just an annoyance. Such type of interference can easily prevent the measurement of sensitive signals in a given microstrip.
One conventional technique which is used in the electronic industry involves multi-layer printed circuit boards in which several printed circuit boards are sandwiched together, one on top of another. Although such an arrangement does provide a certain amount of shielding of the microstrip transmission lines, there are several disadvantages associated with multi-layer systems. For example, the cost of such systems is relatively high. Crosstalk is still a problem due to exposed connector and component pins. Also, there is poor control of characteristic impedance. Further, the multi-layer system is impractical for use at 75 ohm or higher impedance. Moreover, repair of various layers in the multi-layer system is very difficult because of the difficulty of separating the layers without causing damage to the circuits.
It is also possible to obtain shielding by using coaxial cables (i.e., shielded cable), but this requires hand wiring which is very time consuming and expensive. Consequently, this technique normally is used only in small circuits and does not lend itself to practical use in large circuits.
Another technique which has been used involves milling cavities in an aluminum plate and then placing the plate over the printed circuit board. The cost of milling the cavities is quite large, and some form of gasket or conductive adhesive must be used to fill the gaps between the plate and the printed circuit board due to board warp. This technique is practical only for small circuits in low volume production.
Another method for shielding which has been used involves the use of conductive elastomers which are molded and placed over the microstrips. This technique is also very expensive.